May 27, 1994

A weekly feature provided by scientists at the Hawaiian Volcano
Observatory.

The Mauna Ulu Eruption: 1969-1974

Before the current activity on Kilauea's east rift zone, the most long-lived
rift eruption was that of Mauna Ulu, which began 25 years ago, on May 24, 1969.
This eruption lasted until July 24, 1974. There were two main parts of the
Mauna Ulu eruption, separated by a hiatus between October 1971 and February
1972. The first half of the eruption lasted roughly 2.5 years and produced an
estimated 185 million cubic meters of lava (about 240 million cubic yards). The
second half of the eruption lasted another 2.5 years and produced an additional
160 million cubic meters of lava. The lavas erupted between 1969 and 1974 from
Mauna Ulu, and closely associated vents covered 17.6 square miles and added
about 230 acres of new land to Hawaii.

The eruption at Mauna Ulu began on May 24, 1969, and, like the eruption of
Pu'u 'O'o, the earliest part was characterized by episodes of high fountaining
until December 1969, with fountains reaching heights of nearly 1,800 feet. The
next stage of the eruption was passive effusion of lava from a new fissure just
west of Mauna Ulu, then a new fissure just east of Mauna Ulu near Alae Crater.
This quiet, effusive phase of the eruption was similar to that which took place
at Kupaianaha from July 1986 to February 1992. The main differences are that
Kupaianaha was located farther away from Pu'u 'O'o than the Alae fissure was
from Mauna Ulu, and that this phase of the current eruption lasted much
longer.

Starting in June 1971, the surface of the lava pond inside Mauna Ulu began to
drop, and the summit area of Kilauea simultaneously began to inflate (uplift)
as additional magma entered the summit magma reservoir. A brief eruption took
place inside the summit caldera just east of Halemaumau during August 1971, and
another took place inside the caldera and along the southwest rift zone in late
September 1971. By October 15, lava could no longer be seen inside Mauna Ulu
Crater, and the first part of the Mauna Ulu eruption was over. However, magma
continued to enter the volcano, as seen by rapid inflation of the summit
region. No eruptions took place for 3.5 months, but by February 3, 1972, lava
quietly reentered the Mauna Ulu Crater and the second half of the eruption was
under way.

During the 1972-1974 part of the eruption, activity was confined to Mauna Ulu
and a satellitic shield at the former site of Alae Crater, except for two
fissure eruptions along the upper east rift zone - - one near Pauahi Crater in
May 1973 and the other near Pauahi and Hiiaka Craters in November and December
1973. An eruption in the uppermost east rift zone near Keanakakoi Crater and
within the summit caldera from July 19 to 22 marked the end of the Mauna Ulu
eruption, but not the end of activity at Kilauea, since there followed a second
summit eruption in September and a large southwest rift eruption that started
on the last day of 1974. After each of these eruptions, the summit of Kilauea
again inflated as magma continued to arrive and swell the magma reservoir
beneath the summit. The summit deflated catastrophically during a magnitude-7.2
earthquake that occurred on November 29, 1975. This earthquake moved the south
flank seaward and created a large underground volume in which additional magma
arriving from below could be stored. Eruptive activity did not resume on
Kilauea until 1977.

The Mauna Ulu eruption is important because it is our best (and only!)
previous example of a long-lived rift eruption. The sequence of events that
occurred at Mauna Ulu provides us with some insight into how the current
eruption may eventually end.

The current eruption is the most long-lived rift eruption on Kilauea in
historic time; it began on January 3, 1983 and is now in its twelfth year of
nearly continuous activity. Several major phases of the eruptive activity,
starting with an initial period lasting from the start of the eruption until
July 1986, characterized by periodic high-fountaining events built the Pu'u
'O'o cinder and spatter cone.

This phase of activity was followed by development of a new vent, Kupaianaha,
located several miles downrift from Pu'u 'O'o, which passively issued tube-fed
pahoehoe flows from July 1986 until February 1992. These flows covered a large
area, including the village of Kalapana, and entered the sea for several years.
Since late 1991, five additional vents have formed, with the last four located
on the south and west flanks of the Pu'u 'O'o cone. The first was a fissure
eruption between Kupaianaha and Pu'u 'O'o. The currently active vents have
erupted primarily tube-fed pahoehoe flows that have entered the ocean nearly
continuously since November 1992. During the nearly 11.4 years of the current
eruption, approximately 1 cubic kilometer of lava have erupted, about 34 square
miles of land has been covered by lava, and about 500 acres of new land have
been added to Hawaii.

There are similarities between events at Mauna Ulu and those at Pu'u
'O'o-Kupaianaha. Each eruption began with a period of high fountains, and each
evolved into a passive, effusive eruption. The first half of the Mauna Ulu
eruption ended following a decrease in eruptive output and a decrease in the
level of the lava pond in Mauna Ulu, despite the continued addition of magma
into the summit reservoir. When Kupaianaha shut down in 1992, the lava volume
also decreased slowly over time, and the summit of Kupaianaha collapsed
(indicating a withdrawal of magma and drop in the magma level under the
crusted-over vent), despite the continued addition of magma into the summit
reservoir. In addition to the inflation seen at the summit, the lava pond
inside Pu'u 'O'o rose. The shutdown at Kupaianaha after slightly more than nine
years of eruptive activity may be similar to the end of the first half of the
Mauna Ulu eruption that lasted 2.5 years.

Based on the events at Mauna Ulu and at Kupaianaha, we suspect that the end of
activity at Pu'u 'O'o will be heralded by a steady decline in lava output
coupled with a lowering of the lava pond. However, we also expect the summit to
reinflate with magma as the activity at Pu'u 'O'o wanes. Such pressurization of
the summit region could result in eruptive outbreaks further up the east rift
zone from Pu'u 'O'o, at Kilauea's summit, in the southwest rift zone, or in any
combination of these locations. Eruptions occurred at all three places as Mauna
Ulu activity waned in 1974. However, at the present time, we see no signs
suggesting that the current eruption is slowing down.

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26 March 1998